4'-Thio-5-fluorouradine will be tested against several cancer cell lines and a further portion will be submitted to NIH for evaluation of anti-tumor activity. 4'-Thio-cyclic-cytosine will be made and its activity compared with the normal oxygen nucleoside. UDPG with sulfur replacing the ring oxygen in the D-glucopyranosyl unit will be made and tested as a substrate for transferases and for other enzymes using UDPG as substrate.